This invention pertains to the art of continuous casting and more particularly to continuous casting of solid rods, although it is also applicable to the casting of tubing and other shapes.
The invention is particularly applicable to method and apparatus for effecting a fine grain structure in continuous cast copper alloy rods and will be described with particular reference thereto. However, it will be appreciated by those skilled in the art that the invention has broader applications and could be adapted to use for other alloys or materials in other environments.
In continuous casting of solid rods, one type of general casting system employed utilizes a stationary die wherein the casting is intermittently moved generally longitudinally in order to effect the required casting conditions. During a so-called withdrawal stroke, the casting moves fast enough so that only liquid metal enters the cooled length of the die for causing intimate die-metal contact. This stroke is followed by a dwell period during which the casting is stopped or slowed down so that it will exit from the solidification zone at the proper temperature.
Heretofore, in continuous casting of copper and copper alloy rods using the above described as well as other techniques, gross directional solidification occurred in the rod during alloy transition from the liquid to the solid state. Such gross directional solidification results in the development of crystals or grains which grow generally opposite to the direction of heat flow. The grains are usually quite long in the direction of casting and coarse to fine at right angles to that direction. As a result, cold drawing or working of continuous cast copper alloy rods has previously been very difficult or impossible due to the thick grain boundaries generally associated with these structures. Attempts at such cold drawing have undesirably caused cracks and imperfections to appear in the rods as a result of the coarse grains. This, in turn, generates scrap and/or unacceptable end products.
Moreover, if the liquid metal is too hot and/or the casting speed too slow, the grain structure takes on a coarse elongated configuration generally in the direction of casting. Such grain structure is wholly undesirable for metals which are to be subsequently cold drawn. Thus, and within normal casting practice, it is possible to vary the casting parameters so as to alter the grain structure of the cast rod. However, it is not possible to alter these parameters by an amount or to the degree necessary to effect a grain structure which is readily conducive to cold working.
It is known to be beneficial to all casting and metal working schemes to have the grain boundaries be as thin as possible. For this reason, it has been considered desirable to develop an arrangement which would readily facilitate obtaining such fine grain structures in continuous cast copper alloy rods. This result would then satisfactorily accommodate subsequent cold drawing or working of the cast rods. The subject invention is believed to fully meet these desires and provide a substantial, worthwhile improvement by way of both method and apparatus.